1. Field of the Invention
The present invention relates to a plasma processing apparatus. Particularly, it relates to a plasma processing apparatus which can suppress an influence caused by change of a process condition occurring with the progress of plasma processing.
2. Description of the Background Art
For example, the plasma processing apparatus is an apparatus for importing an etching gas into a vacuum process chamber, generating plasma discharge in the imported etching gas under a reduced pressure to thereby generate radicals or ions, and inducing the radicals or ions to a surface of a wafer as a subject of processing to make the radicals or ions react with the surface of the wafer to thereby etch the surface of the wafer.
With the advance of reduction in size of a produced device, such a plasma processing apparatus might not be able to obtain desired performance because of various disturbances even when a predetermined recipe was used for processing.
Therefore, run-to-run control capable of suppressing an influence caused by various disturbances has been used. Run-to-run control is a technique for changing a recipe as a production condition in accordance with each wafer or lot to be processed so that an influence caused by change of process conditions can be suppressed.
For example, JP-A-2003-17471 has disclosed a plasma processing apparatus for processing a specimen contained in a vacuum process chamber, which includes a sensor for monitoring a process quantity during processing, a processed result estimation model for estimating a processed result, and an optimum recipe calculation model for calculating an optimum recipe based on the result estimated by the processed result estimation model, wherein plasma processing is controlled based on the recipe generated by the optimum recipe calculation model.
On the other hand, JP-A-2006-72791 has disclosed a model predictive control apparatus for predicting a subject of control by using control subject models and evaluating the prediction to perform optimum control on the subject of control, which includes control subject models with different sampling periods, wherein one of the control subject models is selected in accordance with change of the sampling period so that both shortening of arithmetic processing time and securement of prediction accuracy can be achieved.
In a plasma etching apparatus, an etching process is generally performed based on a predetermined process condition called recipe. Etching performance (etching rate, etching size, etc.) however often varies in accordance with change of the state of a reaction product deposited on an inner wall or the like of a process chamber, the wear-out degree of each component, etc. To reduce such variations, run-to-run control for changing the process condition in accordance with each wafer to be processed may be used as described above.
Etching rate and processing size are indices for judging whether the result processed by the etching apparatus is good or not. It is however necessary to convey the wafer to an inspection device for measuring the etching rate or processing size. For this reason, a unit capable of evaluating the processed result (performance result) immediately after processing is required for achieving run-to-run control in accordance with each wafer to be processed.
Assume now that the performance result is not directly measured but indirectly measured based on data which can be monitored during processing such as plasma light emission. When, for example, the relation between a monitor value and a performance result is formed as a model in advance, the model can be referred to so that the monitor value can be used in place of the performance result.
Incidentally, for achievement of such run-to-run control, it is necessary to generate a control model by modeling the relation between a process monitor value clearly associated with a processed result obtained in accordance with each wafer to be processed and a control variable capable of controlling the process monitor value.
FIG. 2 is a graph of a control model generated by modeling the relation between a monitor value of plasma light emission during an etching process and a gas flow rate as a process condition. This model shows the relation between the flow rate of an oxygen gas as a gas imported during the etching process and the light emission intensity of a wavelength indicating oxygen as the monitor value of plasma light emission intensity obtained during the process.
In FIG. 2, the reference numeral 1 designates a value which indicates plasma light emission intensity when the process (etching process) is executed without any trouble and which is a target value for run-to-run control. That is, when plasma light emission intensity takes the target value 1, for example, the processing size as an etching performance result is a desired size.
On the other hand, when plasma light emission during the process takes a larger value or a smaller value than the target value 1, the etching performance result is not a desired value because the value of plasma light emission indicates some change of the process condition.
Run-to-run control operates to bring the performance result close to a target value when the performance result is likely to go out of the desired value. That is, in run-to-run control, control quantity for a next process (e.g. next wafer process) is calculated based on a difference between the obtained process monitor value and the target value 1 and a process condition (recipe) for the next process is corrected based on the calculated control quantity so that the process is executed in the corrected process condition.
FIG. 3 is a graph showing plasma light emission intensity obtained by execution of run-to-run control using the aforementioned control model when values of the plasma light emission intensity are plotted in accordance with wafer processes. In FIG. 3, the reference numeral 2 designates a time point that a product lot to be processed is changed.
In this example, plasma light emission intensity is designed to converge at the target value 1 by run-to-run control. There may be however the case where it impossible to perform control to make the plasma light emission intensity coincident with the target value 1. In practice, the plasma light emission intensity is controlled with some variation 3 as shown in the example of FIG. 3. Because this variation means that the performance result varies, this variation exerts an influence on performance of produced products. Such a variation is caused by the fact that the control model cannot reflect all process conditions occurring in the process chamber.
Incidentally, the example disclosed in JP-A-2003-17471 cannot be applied to such a long-term process change that the process condition changes in a lot or between lots. In addition, in the example disclosed in JP-A-2006-72791, the control subject models with different sampling periods cannot be applied to such a process that the process condition changes in accordance with each term (in a lot or between lots).
The invention is accomplished in consideration of the aforementioned problems. An object of the invention is to provide a plasma processing apparatus which can execute run-to-run control in which the process condition of the apparatus is reflected, and which can obtain stable performance results.